1. Field of the Invention
The present invention relates to a liquid crystal display panel, and more particularly, to a method for manufacturing a multidomain liquid crystal display panel.
2. Discussion of the Related Art
Generally, a liquid crystal of a liquid crystal display (LCD) panel includes anisotropic molecules. The average direction of the molecules' long axes is referred to as a director. The director distribution in the liquid crystal is determined by an anchoring energy of the liquid crystal on a substrate, and is characterized by an orientation of the director corresponding to a minimum of a surface energy of the liquid crystal and the anchoring energy. Reorientation of the director is achieved by applying an external electric field to the liquid crystal.
A basic unit of an LCD panel includes two glass substrates with the liquid crystal positioned between them. To obtain uniform brightness and a high contrast ratio in the LCD panel, it is necessary to homogeneously align the liquid crystal in a liquid crystal cell.
The most common technique for achieving homogeneous alignment includes forming microgrooves on a surface of a polymer. This results in a strong anchoring energy that provides a stable liquid crystal alignment. This technique is called a rubbing method, which is a simple method of rubbing a polymer-coated substrate with a cloth. The rubbing method is widely used, since it is fast and allows for large scale manufacturing.
The rubbing method, however, has several serious drawbacks. Because the shape of the microgrooves formed on the alignment layer depends on the rubbing cloth and a rubbing intensity, the alignment of the liquid crystal is often heterogeneous, causing phase distortion and light scattering. Further, electrostatic discharge (ESD) due to rubbing of a polymer surface generates dust contamination in an active matrix LCD panel, decreasing production yield and damaging the substrate.
In order to solve these problems, a photo-alignment method has been proposed using polarized ultraviolet light irradiated onto a photosensitive polymer (M. Schadt et al., Jpn. J. Appl. Phys., 31 (1992) 2155; T. Ya. Marusii & Yu. A. Reznikov, Mol. Mat., 3 (1993) 161). The alignment capability of a photosensitive polymer is determined by an anisotropy of the photosensitive polymer, which is induced by ultraviolet light irradiation.
Photo-alignment materials used in the LCD panels include polyvinylcinnamate (PVCN) (M. Schadt et al., Jpn. J. Appl. Phys., 31 (1992) 2155), polysiloxane (PS), polyimide (PI), and the like. The photo-alignment properties of these materials appear after the ultraviolet light irradiation by a polarized light.
Moreover, these materials may be used on a base of a silicon polyimide copolymer doped with a diazodiamine dye, or on a base of a polymerizable nematic liquid crystal monomer composition for optical storage (W. M. Gibbons et al., Nature, 351 (1991) 49; P. J. Shannon et al., Nature, 368 (1994) 532). These materials are photosensitive in a visible spectrum.
While the alignment direction of the alignment layer is usually perpendicular to a polarization direction of the ultraviolet light used for irradiation, some materials provide an alignment direction parallel to the polarization direction of the ultraviolet light.
The photo-alignment method has various advantages over the conventional rubbing method. First, there is no electrostatic discharge from a rubbing cloth in rubbing, and therefore, the dust particles do not contaminate the substrate. Second, the photosensitive polymer is able to control the alignment direction and azimuthal anchoring energy of the alignment layer. This, therefore, determines alignment direction in the liquid crystal cell. The photo-alignment method is therefore used to form a multidomain liquid crystal cell, which improves the viewing angle characteristics of the LCD panel.
Several methods are known for forming the multidomain liquid crystal cell using the photo-alignment method.
A method proposed by W. M. Gibbons et al. (Nature, 351 (1991) 49) suggests that after rubbing a photosensitive polymer-coated substrate unidirectionally, the substrate may be irradiated by the ultraviolet light to form an alignment direction perpendicular to the rubbing direction. The LCD panel includes a first substrate, a second substrate rubbed in the same direction as the rubbing direction of the first substrate, and a liquid crystal between the first and second substrates. In the Gibbons method, however, it is necessary to use the rubbing method to make microgrooves in the alignment layer, which still causes problems with the electrostatic discharge and the dust contamination from rubbing.
Modification of the Gibbons method is proposed by P. J. Shannon et al. (Nature, 368 (1994) 532). In the Shannon method, instead of rubbing the alignment layer, the first alignment is performed by irradiating the alignment layer with polarized light. While the Shannon method overcomes the disadvantages of the rubbing method, it has a problem in that in order to change the alignment direction during the manufacturing process, two separate irradiation steps using polarized light are required, with the polarization directions being mutually perpendicular. Several processing steps are therefore needed.